The useful power output and life-expectancy at operational power levels are two metrics for comparing the performance of semiconductor lasers. One short-hand measure of the long-term reliability of the laser device is the electronic overstress (EOS) level. This refers to the light output at which the laser device is irreversibly damaged. In contrast, useful power output is typically measured by the device""s kink power. The useful operating power of laser devices is limited in many applications by a xe2x80x9ckinkxe2x80x9d in the power versus current dependence above the lasing threshold, and weakly-guided semiconductor devices, such as ridge waveguide lasers, are particularly susceptible to these kinks. Kink definitions vary greatly but typically correspond to deviations of approximately 20% from a linear dependence above the threshold.
Various techniques have been employed to optimize EOS levels and kink powers, but typically, tradeoffs are involved. For example, power density along the ridge and output facet heating typically dictate the EOS level since output facet destruction is a common failure mode. Widening the ridge decreases power density for the same current levels along the ridge and increases the size of the facet to thereby similarly decrease heating per unit area, increasing the EOS level.
Wider ridges, however, are more susceptible to kinks. A number of different theories have been proposed to explain the kink in the power vs. current dependence. The theories agree insofar as there appears to be a shift of the eigenmode space at the higher currents that affects the total optical output and/or how the output is coupled into a fiber transmission media. Generally, wider ridges are less capable of preventing these shifts.
Another technique for raising the EOS level is sometimes referred to as the E2 process. This involves the vacuum cleaving, i.e., in an oxygen-free atmosphere, followed by the deposition of a thin, silicon layer on the output facet. The underlying theory is that corrosion and electrical surface traps, resulting from crystal defects, dissipate energy at the output facet causing degradation and ultimately catastrophic failure. The silicon layer undermines both of these mechanisms, but the increase in performance is typically less than anticipated since the silicon layer itself is absorptive. Therefore, with this technique, one mode of power dissipation at the facet is simply substituted for another, albeit less, detrimental mode.
The present invention concerns a semiconductor electro-optical device such as a laser or modulator, preferably a 980 nm pump laser that is used to optically pump a gain fiber such as erbium-doped fiber. It comprises a ridge of active and wave-guiding semiconductor layers extending between two facets. In the preferred embodiment, cavity length is relatively long, i.e., the facets are separated by greater than a millimeter. This lowers the current densities in the ridge for the same output beam power. Further, an oxygen-free passivation layer is formed over at least one of the facets to prevent surface corrosion and avoid electrical surface traps. However, a standing-wave-shifted coating is further used on the facet with the passivation layer to reduce the electric field magnitude in the passivation layer.
The p-metal layer, which is used to conduct a ridge-injection current, is preferably greater than 0.2 xcexcm in thickness. This helps to control thermal lensing and thereby increase kink powers. The layer is preferably evaporated gold followed by gold plating, and can be as thick as 1.5 xcexcm. Further, a tapered ridge is preferably used to protect the facet while controlling kinks.
The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.